In recent years, there has been a demand for high wet grip performance and low rolling resistance in pneumatic tires. In order to satisfy these demands, there is a known technique of compounding a reinforcing filler such as a styrene-butadiene copolymer or silica with a rubber composition constituting a cap tread of a tire. In order to further enhance the wear resistance or rubber hardness of the rubber composition, it has been proposed to compound polybutadiene or a silica having high reactivity, for example, but in this case, there has been a problem in that the rubber strength decreases or the processability is diminished. In addition, it is anticipated that increasing the tensile strength at break of the rubber composition will further increase the breakdown resistance of the pneumatic tire. It is also anticipated that increasing the tensile elongation at break of the rubber composition will further increase the durability of the pneumatic tire. However, it has been difficult to achieve good tensile strength at break and tensile elongation at break at a high level.
Japanese Unexamined Patent Application Publication No. 03-239737 describes that a pneumatic tire using a rubber composition prepared by compounding a silica and a styrene-butadiene copolymer having a specific arrangement of styrene-derived units for a tread simultaneously achieves wet skid resistance, rolling resistance, and wear resistance. However, this rubber composition has not necessarily been capable of sufficiently satisfying the demands of consumers due to a decrease in rubber strength.
Japanese Unexamined Patent Application Publication No. 57-179212 describes a styrene-butadiene copolymer in which, a long-chain styrene block content is not greater than 5 wt. %, a simple chain content having one styrene-derived unit is not less than 50 wt. % relative to the total styrene content in the styrene-butadiene copolymer, and a total styrene content is from 10 to 30 wt. % of the styrene-butadiene copolymer content. However, the styrene-butadiene copolymer of Japanese Unexamined Patent Application Publication No. 57-179212 was not sufficient to enhance the rubber composition so as to achieve good tensile strength at break and tensile elongation at break in a compatible manner.